The present invention relates to magnetic recording pulse detection systems and, more particularly, to such systems using peak qualification.
Binary data is stored on magnetic media (disk) by applying a magnetic flux whose polarity reverses with each consecutive data pulse. The analogue signal read back from the disk consists of a series of peaks corresponding to the flux reversals on the magnetic medium which represent the stored original binary data.
For error free reconstruction of the data, the time position of the peak must be found and a digital output pulse produced in response with no change in the timing relationships. The detection techniques are complicated because the shape of the analogue signal varies with different heads, media, head/track alignment and read circuitry non-linearity. Additionally, the stored bit density increases towards the inner tracks and so the signal amplitude decreases with significant bit-interaction, resulting in more distortion and bit shifting.
Typical encoding techniques such as MFM (Modified Frequency Modulation) contain mainly three frequency components, f, 1.5 f and 2 f. Since consecutive data pulses cause flux reversals, the analogue read signal maximum frequency is always half that of the binary data. Optimally, the recorded waveform is adjusted so that at maximum frequency, 2 f, it is sinusoidal. However, at lower frequencies the waveform will go through points of inflexion, or shoulders, which cause peak detection problems.
A typical pulse detector is shown in FIG. 1 and consists of a wide-band amplifier 10, a low-pass filter 12, a differentiator 14, a zero-crossing comparator 16 and a bi-directional pulse generator 20. After amplification and low-pass filtering, the signal is differentiated to find the peaks. The zero-crossing caused by the differentiated peak is located by comparator 16 and an output pulse is generated by pulse generator 20. A flip-flop 18 may be added as discussed below. The input to pulse generator 20 is bi-directional so that both flux polarities are found.
When the waveform is optimal (as shown in the first part of FIG. 3), this technique is simple and economic. However, when the waveform is not optimal, many errors will occur due to false triggering. The quality of the waveform and its associated errors fall into three areas (also shown in FIG. 3):
Area I: Lack of amplitude.
The wave-form in this area exhibits shouldering and is primarily composed of the sum of the fundamental frequency, f, and its 3rd harmonic, 3 f. The frequency response of the amplifier and particularly the linearity of its phase response further affect the shape. The read damping, optimized to the 2 f frequency, will tend to cause over-shoot leading to false peaks at lower frequencies.
Area II: Double peaking close to each other.
Overrange signals in this area are clipped, resulting in peak-distortion, or ringing. This, combined with non-linearity in the amplitude and phase responses, will shift the peak and deteriorate the error margin. This can result in an unacceptable multi-triggering output.
Area III: High frequency spiking noise.
This area is characterized by high energy spikes. These can come from several sources, such as surface defects (e.g. dirt or dents on the disk) read back and passed through a poorly designed band-pass filter. Other sources could be substrate interference or feed-through from digital circuitry on the same chip. Electrical or magnetic spikes can also be received through the package leads, especially at differential pins. As long as it has sufficient energy, a spike can produce two spurious pulses.
The errors produced as discussed above are typically dealt with by requiring each peak to be of a minimum amplitude before it is considered data. Such a system is shown in U.S. Pat. No. 4,081,756 to Price et al. Besides the basic detection circuitry, this method qualifies the peaks depending on whether their magnitudes are above a certain threshold with a threshold comparator 22 as shown in FIG. 1. Threshold comparator 22 has hysteresis and has a toggle output which changes according to whether the peak has sufficient amplitude to pass through either the positive or negative threshold from the zero level. Data latch 18 following the comparator thus uses its output to gate through only one pulse per peak cycle to prevent multi-triggering in Areas I and II. However, in Area III short spikes may still have sufficient amplitude to cause false triggering.
Another possible method for eliminating erroneous peaks would be to use time domain filtering. This method would qualify the signal peaks by insuring that there is a minimum period between them. After the zero-crossing detector the signal can be delayed with respect to itself and used to clock the data latch. Only peaks with sufficient spacing will pass through. Thus, those in Area III will be screened. Area II-type ringing peaks will be screened as long as the ringing has settled within the delay period. However, this method will not be effective enough to guarantee the screening of distorted waveforms such as those in Area I despite their normally small amplitude.